Internal combustion engines are known to include exhaust gas recirculation (EGR) systems to reduce NOx (nitrous oxide) emissions. Air enters the engine through a turbocharger through a compressor, which pressurizes the air. The pressurized air flows to an intake manifold and enters the cylinders of the engine. The compressor is coupled to a turbine, which is driven by exhaust gas from the cylinders. The exhaust gas from the cylinders enters an exhaust manifold and flows into the turbine. The exhaust gas exits the turbine and is vented to the atmosphere. A fraction of the exhaust gas is diverted from entering the turbine and routed back to the intake manifold. The resultant air charge to the cylinder contains both fresh air and combusted exhaust gas.
It is desirable in the industry to improve EGR flow rate to reduce engine emissions while maintaining reasonable fuel economy performance. In order to achieve the desired exhaust gas flow through the EGR system and into the intake manifold, the pressure in the exhaust manifold must be higher than the (boost) pressure in the intake manifold. At times, the average boost pressure at the intake manifold is close to or higher than the back pressure, making flow through the EGR system negligible or non-existent during these times. Further, when the boost pressure is higher than the exhaust pressure, backflow from the intake manifold to the exhaust system results when the EGR valve is open.
A common approach to increasing pressure differential between the exhaust system and the intake system is to rematch the turbocharger. With a good match of the turbocharger, the exhaust manifold pressure may be higher than the intake manifold pressure. Nevertheless, matching techniques do not provide desired EGR mass flow under all engine conditions. Too much back pressure may negatively impact engine fuel economy.
Accordingly, there is a need for a method and apparatus that provides improved EGR mass flow rate to the intake manifold.